In the field known as “hydroforming”, it is known to use a pressurized liquid injected inside a preform placed in a mold to shape a container according to the shape of the mold and fill said shaped container with the liquid at the same time. Advantageously, the injected liquid is the end product contained in the container, i.e. the product which is intended to be provided to a consumer using the container.
For allowing the deformation of the preform into a container, the preform is heated at a temperature greater than the glass transition temperature and lower than the crystallization temperature of the material of the preform such that the preform is placed in a malleable state and is able to expand up to the shape of the container to be produced.
The liquid injected in the preform is generally injected at a temperature lower than the glass transition temperature. The temperature of the injected liquid is for example the ambient temperature, typically from 5° C. up to 50° C. while the glass transition temperature is for example over 75° C. for PET. Consequently, the liquid causes a rapid quenching of a very thin section of the inner walls of the preform during the injection of the liquid, generating a huge shear of temperature gradient with the outer wall of the preform.
Therefore, during the deformation of the preform while the container is not fully shaped, if the container is cooled down to a temperature under the glass transition temperature, the material of the container is no longer malleable and it is possible that the container can no longer be expanded up to the wanted shape or that the obtained container having a part deformed at a temperature below the glass transition temperature is not satisfactory in terms of mechanical and/or thermal resistance and/or in terms of gas permeability.
In order to solve this issue, the temperature of the preform can be increased such that the moment at which the temperature of the material of the container becomes inferior to the glass transition temperature is delayed in order for this moment to occur after the time needed to completely shape the container.
However, increasing the heating temperature of the preform close to the crystallization temperature has several drawbacks.
The heating of the preform at a high temperature releases the internal stress in the material of the preform which can cause an unwanted deformation of the preform prior to its shaping into a container. This deformation is for example a bending of the preform, known as the “banana effect” and/or a height variation of the preform. Such an unwanted deformation of the preform can lead to a malformation of the container which is susceptible of bursting inside the mold in which the container is produced. In this case, the liquid filling the container is spilled in the mold and in the machine for forming the containers and soils the machine. The machine than has to be stopped and cleaned and the destroyed container has to be removed from the machine. The throughput of the machine can therefore be greatly reduced. Furthermore, the heating of the preform at a higher temperature implies a greater energy consumption, which impairs the energy balance of the machine.
US-2014/157726 discloses a hydroforming method using a sensor unit, which can be arranged to vibrate the wall of the preform during the injection of liquid such that the characteristics of the vibrations are captured to determine if a container is leaking or if a burst of the container has occurred. This documents that vibrating the preform wall is not necessary if a very sensitive vibration sensor is used since in this case the natural vibrations of the wall of the preform can be captured.
WO-95/22447 discloses a mold unit comprising a vibrating wall arranged to vibrate the material of the container applied against the wall, meaning that the vibrations occur after the forming of the container, once the container is already shaped and in contact with the wall of the mold. The vibrations are arranged to improve the physical properties of the molded container.
However, these documents do not address to problem of the temperature of the wall of the preform during the forming step.
One of the aims of the invention is to solve the above-mentioned drawbacks by proposing a hydroforming method allowing the container to be properly shaped without requiring an overheating of the preform.